John McDonnell participated in a panel debate about the success of democracy at Glastonbury.
Photograph: Hadley Freeman for the Guardian

John McDonnell has been to Glastonbury a few times but he was particularly disappointed about having to cancel his trip to Worthy Farm last year.

“We were supposed to be here but a coup occurred,” the shadow chancellor, dressed down in jeans, a polo shirt and a blazer, recalled backstage at the Left Field tent. “People started resigning. I felt like phoning up Hilary Benn and saying, ‘could you not put off the coup for a week?’ The timing was dreadful.”

But a lot can change in a year and not only has Jeremy Corbyn now reduced the Conservatives’ Commons majority in the general election, but this weekend he also drew tens of thousands of people to the main Pyramid Stage of Glastonbury for a speech outlining his vision of a fairer society.

“The response to Jeremy was fantastic,” McDonnell said. “We weren’t expecting that. I spoke at the Pyramid Stage about 12 years ago, and within 10 seconds I cleared the field.

“Paul McCartney was here that year, I’m an old Beatles fan. I remember we were knee-deep in mud. As I spoke on stage, my son, who must have been eight or nine then, was sat at the side being looked after by the Scissor Sisters and Black Eyed Peas. They were excellent child minders.”

McDonnell and Corbyn have repeatedly spoken about how Labour’s politics of hope has beaten the politics of fear. How then did Labour plan on keeping the young people who supported it engaged, without disappointing or disenfranchising them because it doesn’t have the power to put policies in place?

“We keep on campaigning the way we’re doing,” he said. “The general election campaign consisted of Jeremy touring all around the country, doing meeting after meeting, we’re now rolling that out to the whole of the shadow cabinet, right the way through the summer and into the winter, expecting a general election at any time.

“We’ll try and ensure there’s mechanisms for people to engage with us in the discussion about their community. The manifesto wasn’t a completed work, it was for the election itself, so now we broaden and deepen that. In some ways the election interrupted our work, but we’ll continue to talk to people about their lives and the policies they need, and then draw that into practical and pragmatic conclusions. It’s just really exciting.”

If you look at the demographic of the support Labour had attracted, McDonnell added, it was people below the age of 45. This included new voters, those in their 30s and early 40s who may have given up on politics, as well as older people who were dismayed by the Tory manifesto.

I felt like phoning up Hilary Benn and saying ‘could you not put off the coup for a week?’ The timing was dreadful

And the popularity isn’t just limited to the people at Glastonbury, he said. “Jeremy has been doing these meetings all around the country, thousands of people turned up all over. I kept saying to people, once we’ve got balanced broadcast coverage in the election campaign people would see what he’s about: an honest, decent, principled person. People are attracted to him as a result of that.”

At the mention of media coverage in the run-up to polling day McDonnell laughs and shakes his head. He was walking down the street when he found out about the headlines calling him and Corbyn terrorist sympathisers. “You get used to it, it doesn’t surprise me at all anymore,” he said. “That sort of continuous attack by the Mail and Sun and the rightwing press does have its effect, there’s no doubt about it, but in this general election it was so far over the top in terms of criticisms, it was so exaggerated and unfair that people turned off completely.

“And what we’ve never had in the past is an alternative source of news, people have now turned to social media. The latest stat is something like 40% of people get most of their information online. I think the virulence of the attacks was such that it turned people off.”

Knocking on doors in his constituency of Hayes and Harlington, McDonnell found that a lot of people were angry about the coverage, “and it wasn’t necessarily from people who were eventually going to vote for me. There’s something in people’s character, particularly the British character, about unfairness. They don’t like it. And the irony was that the more bitter and exaggerated the attacks on Jeremy, the more he became an underdog, and British people support an underdog.”

McDonnell took part in a panel debate on the Left Field stage on Sunday afternoon about the success of democracy, in which he said politicians’ decisions over recent decades were important factors in the deaths of 79 people in Grenfell Tower.

He also said the Labour party had broken the neoliberal grip on intellectual thought in the country.

“For a long period of time, all economic thought has been dominated by trickle down economics, where if you issue tax cuts to corporations and the rich, the money will somehow filter down the rest of society,” he explained.

“Well that’s significantly been proven wrong in this recent political debate, and we can see it, whether it’s people queuing up at food banks, housing shortages, millions of children living in poverty, two thirds of those families with someone in work. People all around are realising that this economic theory has failed, and what we’re trying to do is offer them an alternative. That’s what our manifesto was all about.”

McDonnell also articulated his vision for what he terms a “political renaissance”, where people aren’t limited by existing ideas and structures and have the ability to come up with ideas that the party can debate and discuss. “We’re trying to open up every avenue we possibly can for people to get engaged. It’s about asking people what are you interested in, how do you think it works better, what ideas have you got? It just overcomes some of the staleness we might have had in past years.”

This, he continued, was less idealistic than it sounded. “When you talk to people about their practical life, for example when they’re at work, like the rail industry, the RMT members know better than anyone else how to run their industry. This argument is that if you let the workers run it themselves it would just be self-interest, it isn’t, there’s a hell of a lot of altruism out there.”

McDonnell said Labour was trying to bring a bit of stability into the shadow cabinet after a number of senior members resigned last year. “Jeremy had all those people resign from the shadow cabinet last year, so he had to bring all the youngsters on, and they’ve just been unbelievable. People like Rebecca Long-Bailey, Angela Rayner, Richard Burgon, superb, they’re now trying to dig themselves into a brief so they know it when we go into government.

“We’re trying to do a training exercise where we’re bringing in people like Bob Kerslake, the head of the civil service, to work with them on things like what it would be like to be a minister, what their first day will be like, what their first 100 days will be like. So they feel confident about that as well.”

The TUC joined senior Labour party figures on Sunday in urging the leadership to support a softer form of Brexit. “We don’t go into the hard and soft Brexit definition,” McDonnell said in response. “The issue for us has always been the TUC’s position, which is protecting jobs and the economy. That’s always been our priority. The structures to achieve that is secondary to the objectives themselves.

If you learn a lesson about how you negotiate, it’s that it is based upon mutual interest and mutual respect

“In the two years that Jeremy’s been leader he’s toured around Europe, he’s been meeting with socialist and social democratic leaders and other progressive leaders as well – he’s established a good working relationship. In my old job, when I was chief executive of the Association of London Authorities, we had an office in Brussels. I managed the European funds for London, and if you learn a lesson about how you negotiate, particularly with European leaders, it’s that it is based upon mutual interest and mutual respect. What you don’t do is what Theresa May did and threaten to walk out. If we were in government now the first thing we would do is change the tone of those negotiations by guaranteeing the rights of EU citizens.”

McDonnell also commended the creativity of activists and Labour supporters during the election campaign. “Did you see that game, Corbyn Run? Incredible really. It’s Jeremy running, it’s a bit based on Mélenchon, he shakes the money out of people, out of bankers, and if you score a certain amount you can then launch a new policy,” he said, revealing that he’s played it more than once himself.

And does he ever get the Jeremy Corbyn song stuck in his head? “Yes. We keep bursting out with it. It’s extraordinary.” Before bed? “That’s not part of the renaissance,” he laughs.

The shadow chancellor’s music taste is broad, and he was looking forward to seeing Laura Marling later. Stormzy, he added, was “fantastic last night”, and Alison Moyet was “unbelievable, she held the audience, it was just an incredible set”.

He was also spotted at Craig David. “I was waiting for Jeremy to come on! Actually, he was all right. He’s sort of relaunched hasn’t he, there was quite a warm feeling for him where it was very critical before – he’s been through it, poor bloke.”

Despite the time-intensive process of smashing through neoliberalism, McDonnell tries to make time for interests outside of work.

“My ambition is to learn to play the trombone,” he said. “My wife pulls my leg about it. I’ll find time, my neighbours might not appreciate it but I’m going to try.

“Jeremy’s big thing is all the kids should learn a musical instrument. My musical career ended at primary school when I stood next to the strongest boy in class and he straightened my triangle.”

And what has the shadow chancellor previously done that he would like to say he hadn’t?

“I sneaked into an Everton match once,” he said. “I’m a Liverpool supporter, but Liverpool were away, Liverpool reserves weren’t playing, there wasn’t even a youth match, so I took my son into an Everton match. God help me. It wasn’t me.”